1. Field of the Invention
This invention relates to an improvement in data processing systems such as a microfilm handling apparatus, wherein optical-mechanical image storage and readout devices automatically locate and position a predetermined image frame of a microfilm web in a viewing station.
2. Description of the Prior Art
The improved microfilm handling apparatus of the present invention has certain features which are common to the prior art. The microfilm is contained on supply and takeup reels which are supported by rotatable supporting members, such as spindles, so that the film web may be transported through a viewing station. The viewing station includes a light source and optical elements defining a focal plane in which the predetermined microfilm image frame is positioned for projection of an image therefrom. The reels are driven in accordance with currents furnished from a power supply system. The power supply system is controlled by an error signal provided by a comparing network. The error signal is produced in response to a comparison of a web motion signal indicative of the actual web transport speed and direction, with a control signal indicative of the desired web transport speed and direction. The web motion signal is provided by a sensing device and the control signal is provided by a control device. The predetermined image frame is located in the viewing station by counting position markers which may be located on one edge of the web and adjacent each image frame, and by providing control signals in accordance with such count. The marker count is provided by light detectors such as photocells positioned within the viewing station adjacent the focal plane.
The typical prior art microfilm handling apparatus having means for controlling microfilm web motion and for locating a predetermined microfilm image frame in a viewing station, as described above, are shown in U.S. Pat. Nos. 3,184,177, 3,290,987, 3,596,253 and 3,802,771 and in German Pat. No. 1,270,597.
In particular, U.S. Pat. No. 3,802,771 discloses a microfilm web handling apparatus using dual photocells which abut each other and which have an effective sensing diameter at the focal plane which is not greater than one-half the dimension of a position marker parallel to the direction of travel of the web. The threshold level at which signals from the dual photocells are sensed to indicate the presence of a position marker is automatically adjusted upon insertion of a microfilm web into the apparatus, thereby compensating for any variations in component tolerances, ambient conditions or variations in the overall optical density of the film. Such an apparatus is not capable of changing the threshold level in response to variations in optical density as may occur along a web or from one edge of a web to another or to variations in the ambient level resulting from line voltage variations and the like. Furthermore, that apparatus is incapable of sensing position markers positioned adjacent image frames arranged in two parallel tracks along the web and of transversely positioning image frames in either track with respect to the viewing station.